The show is set in a fictional continent called the "Land of Ooo" that,[10] takes place in a post-apocalyptic world some time after a nuclear war called the "Great Mushroom War." According to Ward, the show takes place "after the bombs have fallen and magic has come back into the world".[11] In "The Real You", when Finn wears the Glasses of Nerdicon, planet Earth can be briefly seen from space, with a large chunk missing from it. Later it is shown in the episode "Five Short Graybles," which initially shows the storyteller with a rotating holographic globe with a portion missing, where the North American continent would be. During a presentation at the Toronto Comics and Arts Festival, Ward said that this will likely never be directly addressed in the series.

References to the war are shown constantly in the show. The first two seconds of the opening credits show nuclear bombs, melee weapons (such as swords and axes), abandoned human items (such as a TV set, headphones, cassette and video tapes) are scattered everywhere, even a skull on the ground. In "Ocean of Fear", when Finn and Jake are out in the middle of the ocean, there is a tank on the small patch of land they swim to. Later on, when they swim to the bottom of the ocean, there are a many sunken cars and tall sunken buildings.

The war's effect on humans is also shown, with there seeming to be no other full human character except for Finn. However, human remains are scattered all around Ooo. Also, in "Beautopia," a character by the name of Susan Strong is implied to be human as well, although she belongs to a strange fish headed, animal hatted mutant tribe. In "Holly Jolly Secrets", Finn and Jake discover a few secret VHS tapes by the Ice King, where the last tape shows Ice King as a human. In the tape, he explains he purchased his golden crown from an old dock worker in northern Scandinavia.